This invention relates to a curable fluoropolyether base rubber composition which is suited for liquid injection molding and cures into products having improved mechanical properties.
In the prior art, linear fluoropolyether compounds containing at least two alkenyl groups per molecule and having a perfluoroalkyl ether structure in their backbone are used in a variety of applications owing to the excellent characteristics of the perfluoroalkyl ether structure. While silica fillers including dry silica (known as fumed silica) and wet silica (known as precipitated silica) are commonly used in silicone rubber for reinforcement purposes, it is known that blending such silica filler in the curable fluoropolyether base rubber can remarkably improve the mechanical properties of the cured product thereof. The blending of silica filler provides fluoropolyether base rubber compositions with a good balance of heat resistance, chemical resistance, solvent resistance, water repellency, oil repellency, and weather resistance. The resultant compositions perform well in most applications.
However, a problem arises in molding such rubber materials. When O-rings and diaphragms for use in semiconductor parts and hard disks are to be molded, liquid injection molding (LIM) featuring mass-scale productivity is often used. A limit is imposed on the viscosity of liquid rubber materials which can be molded. This, in turn, imposes a limit on the permissible loading of reinforcing silica. It is then difficult to find a compromise between good mechanical strength and ease of liquid injection molding.
An object of the invention is to provide a liquid curable fluoropolyether base rubber composition which is amenable to liquid injection molding and cures into products having improved mechanical properties.
The present invention provides a curable fluoropolyether base rubber composition comprising
(A) 100 parts by weight of a linear fluoropolyether compound containing at least two alkenyl groups in a molecule and having a perfluoroalkyl ether structure in its backbone,
(B) 10 to 50 parts by weight of a silica filler having a specific surface area of at least 100 m2/g and a bulk density of 100 to 200 g/l,
(C) an effective amount to cure component (A) of an organosilicon compound having at least two hydrogen atoms each bound to a silicon atom in a molecule, and
(D) a catalytic amount of a hydrosilylation catalyst. This fluoropolyether base rubber composition is prevented from a viscosity rise due to filler loading, is amenable to liquid injection molding, and cures into products having improved mechanical properties.
The respective components of the curable fluoropolyether base rubber composition are described below.
(A) Linear Fluoropolyether Compound
The linear fluoropolyether compound used herein as a base polymer in the composition is one containing at least two alkenyl groups in a molecule and having a divalent perfluoroalkyl ether structure in its backbone.
The alkenyl groups in the linear fluoropolyether compound are those having a CH2xe2x95x90CHxe2x80x94 structure at an end such as vinyl, allyl, propenyl, isopropenyl, butenyl and hexenyl, with the vinyl and allyl being especially preferred. The alkenyl groups may be attached either directly to both ends of the backbone of the linear fluoropolyether compound or to the backbone through a divalent linking group such as xe2x80x94CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94 or xe2x80x94Yxe2x80x94NRxe2x80x94COxe2x80x94. Herein Y is xe2x80x94CH2xe2x80x94 or a group of the following structural formula: 
(the bond may be at o, m or p-position) and R is hydrogen, methyl, phenyl or allyl.
The perfluoroalkyl ether structure in the linear fluoropolyether compound includes those of the following general formula:
xe2x80x94(Rfxe2x80x94O)q 
wherein Rf is a straight or branched perfluoroalkylene group of 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, and letter q is an integer of 1 to 500, preferably 2 to 400, more preferably 10 to 200.
Examples of the recurring units xe2x80x94(Rfxe2x80x94O)xe2x80x94 are shown below.
xe2x80x94CF2Oxe2x80x94, xe2x80x94CF2CF2Oxe2x80x94, xe2x80x94CF2CF2CF2Oxe2x80x94,
xe2x80x94CH(CF3)CF2Oxe2x80x94, xe2x80x94CF2CF2CF2CF2Oxe2x80x94,
xe2x80x94CF2CF2CF2CF2CF2CF2Oxe2x80x94, and xe2x80x94C(CF3)2xe2x80x94.
Of these, xe2x80x94CF2Oxe2x80x94, xe2x80x94CF2CF2Oxe2x80x94, xe2x80x94CF2CF2CF2Oxe2x80x94, and xe2x80x94CH(CF3)CF2Oxe2x80x94 are preferred. It is understood that the perfluoroalkyl ether structure may consist of recurring units xe2x80x94(Rfxe2x80x94O)xe2x80x94 of one type or recurring units of two or more types.
Typical of the linear fluoropolyether compound (A) are those of the following general formula (1). 
In formula (1), X is independently selected from among xe2x80x94CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94 and xe2x80x94Yxe2x80x94NRxe2x80x94COxe2x80x94, letter p is independently equal to 0 or 1, r is an integer of 2 to 6, and m and n are integers of 0 to 200, preferably 5 to 100. Y is xe2x80x94CH2xe2x80x94 or a group of the following structural formula: 
(the bond may be at o, m or p-position), and R is hydrogen, methyl, phenyl or allyl. These linear fluoropolyether compounds have a molecular weight of about 400 to 100,000 and preferably about 1,000 to 50,000.
Illustrative examples of the linear fluoropolyether compound of formula (1) are given below. In the following formulae, m and n are as defined above. 
These linear fluoropolyether compounds may be used alone or in admixture of two or more.
(B) Silica Filler
The filler (B) used for the reinforcement of the curable fluoropolyether base rubber composition is particulate silica. It should have a specific surface area of at least 100 m2/g as measured by the hydrogen adsorption BET method. From the molding standpoint, the silica filler should have a bulk density of 100 to 200 g/l, and preferably 130 to 160 g/l, in order to prevent the composition from increasing its viscosity as a result of blending of the silica filler. Too low a bulk density detracts from reinforcement effects whereas too high a bulk density results in thickening.
The silica filler may be either dry silica known as fumed silica or wet silica known as precipitated silica. The silica filler may be treated with organochlorosilanes, silazane compounds and cyclic silazane compounds which react with hydroxyl groups attached to silicon atoms on silica surfaces. Alternatively, the silica filler may be hydrophobized on the surface with dimethylpolysiloxanes having a low degree of polymerization.
An appropriate amount of the silica filler (B) blended is 10 to 50 parts, and preferably 15 to 30 parts by weight per 100 parts by weight of component (A). Less than 10 parts of component (B) fails to achieve sufficient reinforcement whereas more than 50 parts of component (B) invites an excessive rise of viscosity and is difficult to compound.
(C) Organosilicon Compound
The organosilicon compound (C) functions as a crosslinking agent and chain extender for component (A). Any organosilicon compound is useful as long as it has at least two hydrogen atoms each bound to a silicon atom, that is, hydrosilyl (SiH) groups in a molecule. With the compatibility with and dispersibility in component (A), and uniformity after curing taken into account, organosilicon compounds having at least one monovalent perfluorooxyalkyl group, monovalent perfluoroalkyl group, divalent perfluorooxyalkylene group or divalent perfluoroalkylene group as well as at least two, preferably at least three hydrosilyl groups (or SiH groups) are preferred.
The perfluorooxyalkyl, perfluoroalkyl, perfluorooxy-alkylene and perfluoroalkylene groups include the groups of the following general formulae.
monovalent perfluoroalkyl groups:
CmF2m+1xe2x80x94
m is an integer of 1 to 20, preferably 2 to 10.
divalent perfluoroalkylene groups:
xe2x80x94CmF2mxe2x80x94
m is an integer of 1 to 20, preferably 2 to 10.
monovalent perfluorooxyalkyl groups: 
n is an integer of 1 to 5. divalent perfluorooxyalkylene groups: 
m is an integer of 1 to 50, n is an integer of 1 to 50, and m+n is an integer of 2 to 100.
xe2x80x94(CF2O)mxe2x80x94(CF2CF2O)nxe2x80x94CF2xe2x80x94
m and n each are an integer of 1 to 50.
These perfluoro(oxy)alkyl and perfluoro(oxy)alkylene groups each may be attached either directly to a silicon atom or to a silicon atom through a divalent linking group. The divalent linking group is an alkylene group, arylene group or a mixture thereof, which may further have an ether bond oxygen atom, amide bond or carbonyl bond. Such divalent linking groups of 2 to 12 carbon atoms are preferred. Illustrative examples thereof include
xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2OCH2xe2x80x94,
xe2x80x94CH2CH2CH2xe2x80x94NHxe2x80x94COxe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94N(Ph)xe2x80x94COxe2x80x94,
xe2x80x94CH2CH2CH2xe2x80x94N(CH3)xe2x80x94COxe2x80x94, and xe2x80x94CH2CH2CH2xe2x80x94Oxe2x80x94COxe2x80x94 wherein Ph is phenyl.
In addition to the monovalent organic group containing a monovalent or divalent fluorinated substituent, that is, a perfluoroalkyl, perfluorooxyalkyl, perfluoroalkylene or perfluorooxyalkylene group, the organosilicon compound (C) may have a monovalent substituent attached to a silicon atom. Exemplary monovalent substituents are substituted or unsubstituted hydrocarbon groups of 1 to 20 carbon atoms including alkyl groups such as methyl, ethyl, propyl, butyl, hexyl, cyclohexyl, octyl and decyl; alkenyl groups such as vinyl and allyl; aryl groups such as phenyl, tolyl, and naphthyl; aralkyl groups such as benzyl and phenylethyl; and substituted ones of these groups in which some of the hydrogen atoms are replaced by chlorine atoms, cyano groups or the like, such as chloromethyl, chloropropyl, and cyanoethyl.
The organosilicon compound (C) may be cyclic, chainlike or three-dimensional network. The number of silicon atoms in the molecule of the organosilicon compound is not critical although it desirably has about 2 to about 60 silicon atoms, and especially about 3 to about 30 silicon atoms.
Illustrative examples of the organosilicon compound are given below. They may be used alone or in admixture of two or more. In the formulae, Me is methyl and Ph is phenyl. 
Note that m is an integer of 1 to 20, averaging to 10, and n is an integer of 1 to 10, averaging to 6. 
n is an integer of 1 to 30, m is an integer of 1 to 30, and n+m is 2 to 60, averaging to 2 to 50. 
n is an integer of 1 to 30, m is an integer of 1 to 30, and n+m is 2 to 60, averaging to 2 to 50. 
Note that n is an integer of 1 to 60, averaging at 3 to 50. 
Note that n is an integer of 1 to 60, averaging at 3 to 50. 
Note that n is an integer of 1 to 60, averaging at 3 to 50.
Component (C) is blended in an effective amount to cure component (A). Usually, component (C) having hydrosilyl groups is blended in such an amount that preferably 0.5 to 5 mol, and more preferably 1 to 2 mol of hydrosilyl groups (or SiH) groups may be present per mol of alkenyl groups (e.g., vinyl, allyl or cycloalkenyl) in the entire composition, especially component (A). Less amounts of component (C) may achieve an insufficient degree of crosslinking. Excessive amounts of component (C) may allow chain lengthening to become preferential, inviting short cure, foaming, and losses of heat resistance and compression set. More particularly, the amount of component (C) blended is usually 0.1 to 50 parts by weight per 100 parts by weight of component (A).
(D) Hydrosilylation Catalyst
The hydrosilylation catalyst (D) is preferably selected from transition metals, for example, platinum group metals such as Pt, Rh and Pd, and compounds of transition metals. Most of these compounds are noble metal compounds which are expensive. Platinum compounds are thus used because they are readily available.
Exemplary platinum compounds include chloroplatinic acid, complexes of chloroplatinic acid with olefins such as ethylene, complexes of chloroplatinic acid with alcohols and vinylsiloxanes, and platinum supported on silica, alumina or carbon though not limited thereto. Known platinum group metal compounds other than the platinum compounds include rhodium, ruthenium, iridium, and palladium compounds, for example, RhCl(PPh3)3, RhCl(CO)(PPh3)2, RhCl(C2H4)2, Ru3(CO)12, IrCl(CO)(PPh3)2, and Pd(PPh3)4 wherein Ph denotes phenyl.
The amount of the catalyst used is not critical. A catalytic amount can achieve a desired curing rate. The catalytic amount varies depending on the form and concentration of the catalyst, that is, whether or not the catalyst is supported on a carrier such as silica or alumina and whether or not the catalyst is diluted with a solvent. From the economical aspect and to obtain satisfactory cured products, the platinum group metal compound is preferably added in an amount of 0.1 to 1,000 parts, more preferably 0.1 to 500 parts by weight calculated as the platinum group metal per million parts by weight of the entire curable composition.
Other Components
Insofar as the benefits of the invention are not impaired, various well-known additives may be added to the inventive composition in addition to the above essential components (A) to (D). Such optional additives include regulators of the hydrosilylation catalyst, for example, acetylene alcohols such as 1-ethyl-1-hydroxycyclohexane, 3-methyl-1-butyn-3-ol, 3,5-dimethyl-1-hexyn-3-ol, 3-methyl-1-penten-3-ol, and phenylbutynol, as well as 3-methyl-3-penten-1-yn and 3,5-dimethyl-3-hexen-1-yn; tackifiers, for example, organosiloxanes having alkoxy, epoxy or SiH groups in the molecule such as the compound shown below; pigments such as iron oxide, cerium oxide and carbon black; colorants, dyes, and antioxidants. 
Depending on its application, the rubber composition of the invention is formulated as a single composition having all the essential components (A) to (D) incorporated therein, that is, of one part type. Alternatively, the rubber composition is formulated to two part type, for example, one part containing a part of (A), a part of (B) and (C) and the other part containing the balance of (A), the balance of (B) and (D) whereupon the two parts are mixed on use.
The composition thus obtained is liquid and should preferably have a viscosity of 50 to 2,000 Paxc2x7s at 25xc2x0 C., especially 200 to 1,000 Paxc2x7s at 25xc2x0 C., as measured according to JIS K7117. A viscosity outside the range may impede molding.
The composition of the invention will cure when it is allowed to stand at room temperature or by heating. Often, the composition is preferably cured by heating at a temperature from room temperature (e.g., 10-30xc2x0 C.) to about 180xc2x0 C. for about 5 minutes to about 24 hours.
The composition of the invention can be molded by any conventional method although the well-known liquid injection molding technique is advantageously applicable to the composition having a viscosity within the above-defined range.
The curable fluoropolyether base rubber compositions cure into products having significantly improved mechanical properties as well as water repellency, oil repellency, solvent resistance, chemical resistance, and weather resistance. Thus the compositions are useful in a wider variety of applications and suitable for use as molded rubber parts such as diaphragms and sealing parts (e.g., O-rings, gaskets and grommets) where chemical resistance and oil resistance are required when such parts are used in chemical plants, business machines (e.g., copiers and printers), automotive and aircraft, semiconductor devices, medical equipment, analytic instruments, etc.